Reverse transcripts of the RNA genome of the Bovine Leukemia Virus (BLV) as well as radiolabeled BLV RNA hybridizes to the DNA of tissues from leukemic cattle with the adult form of the disease, but not to bovine thymic lymphoma or normal bovine tissues. Although BLV has many properties in common with members of the family, it can be distinguished from known type C leukemia viruses by a variety of biochemcial criteria.